1. Field of the Invention
The subject invention pertains to the field of thermoplastic honeycomb structural materials. More particularly, the invention pertains to honeycomb fabricated from fiber-reinforced and non-reinforced thermoplastic webs wherein the density and other physical characteristics of portions of the honeycomb are varied during the manufacture of the honeycomb. The invention further pertains to a process for manufacturing such honeycomb materials.
2. Description of the Related Art
Both fiber-reinforced and non-reinforced thermoplastic honeycomb materials have become important structural materials in the transportation and aerospace industries. In addition to their excellent strength to weight ratio, such materials also have electromagnetic properties which allow their permeability and reflectivity to electromagnetic radiation, particularly in the microwave, or radar range, to be varied across a broad spectrum.
However, frequently the load bearing capability of such honeycomb materials must be adjusted within the honeycomb itself. In the past, this has required carefully machining a cavity in the honeycomb and filling the cavity with a honeycomb insert having different structural properties, i.e. greater wall thickness, fiber content, or a different fiber reinforcement. In these machining operations, the honeycomb core must generally be filled with a cell support substance such as a polyethyleneglycol wax following which the honeycomb is cut, in the case of simple shapes, by a sharp knife or saw; or in the case of complex shapes, by computer controlled three dimensional cutting machines as are commonly found in the tooling and machining industries. Following the machining operation, the polyethylene glycol wax is melted and the part repeatedly washed to remove all traces of wax.
These two male/female parts must then be carefully and laboriously bonded together adhesively. The time and cost of such operations together with the possibility of incomplete bonding has limited the use of such structures.
U.S. Pat. No. 3,356,555 to Jackson discloses a batch method of preparing honeycomb core wherein a squirrel cage roller containing hexagonal bars operates in conjunction with a plurality of hexagonal major bars on a fixed bed to provide a corrugated web by the action of compressing a heated thermoplastic web between the advancing squirrel cage bars and the fixed bed bars. The corrugated webs thus produced are then assembled into a honeycomb structure by placing a first corrugated web on the major bars of a bed followed by placing minor bars in the nodes above the web. A second corrugated sheet is then located with its nodes adjacent to the first web's antinodes. An additional layer of minor bars is placed on top and the procedure repeated until the desired honeycomb thickness is achieved. The assembly is then placed between caul plates and platens and heated to form a finished honeycomb product.
In GB-A-2 188 866, a batchwise method of preparing thermoplastic honeycomb is disclosed wherein shaped formers as disclosed by Jackson in U.S. Pat. No. 3,356,555 are disposed between adjacent corrugated thermoplastic sheets, the assembly pressurized, and heated to cause the thermoplastic to fuse. Following the preparation of one multiple cell honeycomb segment by such batchwise operation, the section may be advanced to allow a further section to be laid up and fused. The process described requires pressurizing and heating the entire newly formed honeycomb structure.
The related art processes produce honeycomb cores in batchwise fashion requiring lengthy heating and cooling cycles to process the honeycomb, or by expansion methods which are directed to but limited honeycomb sizes and to honeycombs not having optimal properties. The processes of Jackson '555 and Great Britain '866 require the use of numerous layers of expensive metal formers. For example, a four foot length of 0.125 cell width honeycomb having but a height of four cells would require in excess of 1400 metal formers. Furthermore, such batchwise processes are time consuming, and require pressurizing an entire honeycomb structure to enable thermoplastic fusion and adherence.
Furthermore, because the abutting node and antinode surface of web material is double the thickness of the non-abutting surface, this thickness disparity creates a displacement error in assembling honeycombs using metal formers. Because the error is magnified by each successive cell layer, it is virtually impossible to keep such cores aligned during layup and during cure under pressure. If the formers are made unsymmetrical, i.e. a flattened hexagon, then layup is facilitated, but distortion still occurs during consolidation as the double thickness abutting layers are compressed during fusion to approximately 80 percent of their initial thickness.
In the parlance of one skilled in the art of honeycomb production and usage, the thickness of the honeycomb is considered as the dimension parallel to the corrugations, the length is the direction along the same surface as the corrugations but at a right angle thereto, and the width is the direction across the cells perpendicular to the plane of the corrugations (thickness and length directions).